The present invention relates to a switching device for the electrical measurement of reactive impedances, comprising n branches in parallel with n.gtoreq.1, each branch being formed from a reactive impedance to be measured and a switch. The assembly of switches being subjected to a control signal which successively connects each reactive impedance in parallel across the measuring bridge.
This type of switching device is generally controlled by an electrical signal. It is then designated by the name "electronic switch". An electronic switch allows the measurement of n components in a minimum time without mechanical transfer. Generally, this type of switching device is used for measuring reactive impedances. By reactive impedance is meant a circuit formed either by one or more capacities, or one or more inductances or by a combination of the two, said circuit possibly comprising resistive elements. This measurement is made by means of a measuring bridge of the WIEN or NERNST type. The measurement takes place at different frequencies depending on the type of impedance to be measured. For further details, reference may be made for example to the article entitled "Mesure sur condensateurs" by C. LECLECQ, published by the central laboratory of electrical industries.
In each branch, the switch has two positions; the closed position which electrically connects the first plate of the capacitor to the first end of the measuring bridge, the second plate of the capacitor being connected electrically to the second end of the measuring bridge and the open position, in which the electrical contact beween the first plate and the first end of the bridge is open. Switching over from one position to the other is caused by an electrical signal of a frequency of the order of 5 Hz, which successively closes each branch so that a single switch is closed and the others open.
The ideal switching device has the following characteristics: a zero resistance when closed, infinite resistance when open and infinitely short switching times. For the switches, REED change-over relays are usually used, having a capacity between contacts less than 1.5 pF, which is closest to these characteristics and as measuring bridge, an automatic RLC measuring bridge, well known to those skilled in the art.
However, the capacities between the contacts of the switch introduce parasite capacities disturbing the measurement. This disadvantage is common to all switches and it does not apply especially to REED change-over relays. In fact, when the switch is open, a parasite capacity between the two contacts of the switch is established in series with the capacity to be measured in the branch considered. The capacity of the switch then takes on the equivalent value: EQU equivalent C=[(C.sub.x .multidot.C.sub.p)/(C.sub.x +C.sub.p)].times.n
in which C.sub.x is the capacity to be measured, C.sub.p is the parasite capacity between contacts and n is the branch number. The equivalent capacity of the switching device thus introduced may assume values comparable to the capacities which are to be measured. The equivalent capacity varies as a function of the number and of the value of the components to be measured. In addition, the value of the equivalent capacity varies at each switch-over, each of the branches not having exactly the same characteristics. Furthermore, all the branches of the switching device are not necessarily used during a measurement. In this case, the parasite capacity of each non used branch is not taken into account for the measurement. From which it follows that it is impossible to measure low value capacities accurately without having to make a correction. Using a correction consisting in balancing the measuring bridge before each measuring cycle has already been proposed. However, this means that the advantage of the speed of electronic switching is lost. The resetting of a bridge to zero takes time during which the switching device could effect several measurements.
Use of a computer for automatically subtracting the parasite capacity has also been proposed. However, this computer is not easy to handle, is space-consuming and considerably increases the cost price of the switching device. In addition, in the case where all the branches are not used, it is necessary to inform the computer thereof so that it effects the adequate correction.
Consequently, there does not exist at the present time a low price switching device retaining electronic switching speed.